1. Field of the Invention
The present invention relates to an exhaust gas purification device of an internal combustion engine. In particular, the present invention is suitably applied to an exhaust gas purification system that uses selective catalytic reduction (SCR) for selectively purifying NOx (nitrogen oxides) in exhaust gas by using ammonia as a reducing agent. The above system is known as a urea SCR system since urea aqueous solution is generally used as the reducing agent.
2. Description of Related Art
In recent years, a urea SCR system has been developed and partly adopted in practical use as an exhaust gas purification system that purifies NOx in exhaust gas at a high purification rate in an engine (specifically, a diesel engine) applied to an automobile or the like. A following construction is known as the urea SCR system. That is, in the urea SCR system, a selective reduction NOx catalyst is provided to an exhaust pipe connected to an engine main body, and a urea solution addition valve for adding urea solution (urea aqueous solution) as a NOx reducing agent into the exhaust pipe is provided upstream of the NOx catalyst.
In the above-described system, the urea solution is added into the exhaust pipe by the urea solution addition valve, so NOx in the exhaust gas are selectively reduced and removed on the NOx catalyst. Concerning the reduction of NOx, ammonia (NH3) is generated when the urea solution is hydrolyzed with exhaust heat. The ammonia adsorbs to the NOx catalyst, and a reduction reaction based on the ammonia occurs on the NOx catalyst. Thus, NOx are reduced and purified.
In order to increase a NOx purification rate in the NOx catalyst and to inhibit occurrence of ammonia slip at the same time, it is necessary to supply just enough ammonia to the NOx catalyst. Therefore, there is a proposed technology that estimates ammonia adsorption quantity in a NOx catalyst (i.e., reducing agent adsorption quantity) and that controls urea solution addition quantity of a urea solution addition valve based on the estimate (for example, as described in Patent document 1: JP-A-2003-293737 or Patent document 2: JP-A-2003-293738). More specifically, NOx discharge quantity discharged from the engine and an actual NOx purification rate in the NOx catalyst are calculated. Ammonia consumption quantity in the NOx catalyst is calculated based on the NOx discharge quantity and the actual NOx purification rate. The ammonia adsorption quantity is calculated based on the ammonia consumption quantity and the urea solution addition quantity at the time. The urea solution addition quantity is controlled in accordance with the ammonia adsorption quantity.
It is difficult to directly measure the ammonia adsorption quantity in the NOx catalyst. Therefore, generally, technologies including the above-described conventional technologies employ a construction estimating the ammonia adsorption quantity from a measurement value of a NOx sensor or the like provided in the exhaust pipe.
However, it is thought that the calculation accuracy of the ammonia adsorption quantity falls if the NOx catalyst is brought to a predetermined high temperature state due to temperature increase. That is, it is thought that the limit value of the ammonia adsorption quantity in the NOx catalyst varies in accordance with the catalyst temperature. When the limit value of the ammonia adsorption quantity is small, the ammonia slip tends to occur. Under a condition where the ammonia slip tends to occur, correct calculation of the ammonia adsorption quantity is difficult. Accordingly, there is a concern that the accuracy of the urea solution addition control falls and eventually the NOx purification rate falls.